This application relates to the art of knockouts that are selectively removable from a wall to provide holes therethrough. The application is particularly applicable to eccentric knockouts in electrical boxes and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and can be used in devices other than electrical boxes.
It is common to provide inner and outer knockouts in the wall of an electrical box for selectively connecting conduit of different sizes thereto. When the knockouts are concentric, a small conduit communicating with the hole provided by removing the inner knockout is connected to the box wall by a fitting that clamps only onto the outer knockout. Forces acting on the fitting or conduit can result in displacement of the outer knockout, and this breaks the connection between the conduit and the box. To overcome this problem, it is known to provide eccentric inner and outer knockouts. Regardless of the conduit size used, the fitting attaching the conduit to the box wall always clamps over a substantial arc directly to the box wall. The joint is thereby able to withstand significant forces applied to the fitting or conduit.
With eccentric knockouts of the type described, the frangible connection between the outer knockout and the box wall must be strong enough to prevent displacement of the outer knockout when the inner knockout is removed. At the same time, the frangible connection between the outer knockout and the box wall must be weak enough to allow easy removal of the outer knockout when so desired. When an eccentric outer knockout is circular and completely surrounds a circular inner knockout, the joint for small conduit is not as strong as it could be if the outer knockout did not completely surround the inner knockout. This is because the fitting for small conduit must span a portion of the outer knockout before directly engaging the box wall.